Constant voltage dimmable LED driver

ABSTRACT

A constant voltage dimmable LED (Light Emitting Diode) driver is disclosed that is compatible with all types of dimmers, including conventional phase cut (TRIAC) dimmers, and behaves like a conventional constant voltage driver which can be connected to any size of LED load that has a matching voltage rating. The driver produces a continuous train of pulses for driving the LED load and obtains an averaged measure of the voltage at the AC input for controlling the duty cycle of the continuous train of pulses. Therefore, when the averaged measure of the voltage at the AC input is reduced by a dimmer, the duty cycle reduces, resulting in a dimmed LED. The driver can be created by adding a few components to a conventional wide input range AC-DC converter without or with very little modifications.

FIELD OF THE INVENTION

The embodiments discussed herein relate to a power supply for drivinglight emitting diodes (LEDs).

BACKGROUND OF THE INVENTION

The use of LEDs in lighting applications is rapidly expanding, as theyare much more efficient and have much longer life time than incandescentlamps. Compared with fluorescent lighting systems, LEDs also have anadvantage that they do not contain harmful substances such as mercury.

LEDs are primarily DC devices and usually require a much lower voltagethan the mains AC power provided by utility companies. For this reason,a power supply or “driver” is needed for converting the mains AC powerto a DC voltage or current appropriate for driving the LEDs.

Conventional AC-DC converters can be used as LED drivers. They have beendeveloped and perfected for many years. They provide high efficiency andcan be manufactured in mass quantities with very low cost. However, theyare fundamentally incompatible with conventional dimmers, such as TRIACdimmers, that are designed primarily for incandescent lamps. The reasonis conventional AC-DC converters try to, and in fact, have beenperfected to, regulate the output voltage for a wide range of inputvoltage and load variations. Therefore, when a dimmer is used to reducethe voltage supplied to a conventional AC-DC converter, the converterstill tries to maintain the same output voltage and will not dim thelight, until the input voltage is so low that the converter cannot workanymore, it will abruptly shut off or become unstable.

Converters that are compatible with conventional dimmers (so called“dimmable LED drivers”) can be made using various techniques. Examplesinclude designs described in U.S. Pat. Nos. 7,649,327, 7,852,017,7,609,008, 7,038,399 and U.S. patent applications 20110043129,20110037399, 20110012530, 20100295478, 20100277103, 20080278092,20100134038, 20090295300. However, all these designs require significantdeviations from the design of conventional AC-DC converters and some areso complex as to require the use of microprocessors. Therefore, thesedesigns cannot leverage the well developed and perfected designs ofconventional AC-DC converters. As a result, the currently commerciallyavailable dimmable LED drivers are usually much more expensive thanconventional AC-DC converters of the same wattage. Furthermore, most ofthe commercially available dimmable LED drivers are constant currenttype while a constant voltage type of LED driver is more desirableespecially in flexible LED strip applications.

OBJECT OF THE INVENTION

Applicant recognizes and appreciates that a constant voltage type of LEDdriver is much more desirable than a constant current type especially inflexible LED strip applications. Because LED strips are usuallyspecified by the voltage, while a constant current LED driver requirescarefully matching the current of the driver with the LED strip or theLED strip may be burned out or does not give enough light. Furthermore,Applicant recognizes and appreciates that in many applications, LEDstrips often need to be cut into various lengths during the installationresulting in non-standard current requirements that a matching constantcurrent LED driver simply cannot be found. In contrast, a conventionalconstant voltage AC-DC converter, although not “dimmable”, only requiresmatching of the voltage rating of the LED with the converter as long asthe wattage of the LED does not exceed the maximum wattage of the AC-DCconverter. For example, a 12V 100 W AC-DC converter can be used to drivea 12V 100 W LED strip as well as a 12V 5 W LED strip. Therefore, a 12VLED strip can always be connected to a 12V constant voltage LED driverregardless of what length the LED strip is cut into. Even if the wattageof the LED strip exceeds the maximum wattage of the AC-DC converter, theconverter is usually equipped with over current protection and will beeither current limited or shut down. There is no possibility of damagingeither the LED or the converter, as long as the voltage rating ismatched.

There is a need for a dimmable LED driver that is compatible withconventional dimmers, can leverage the well developed and perfected artof making conventional AC-DC converters, and behaves like a conventionalconstant voltage AC-DC converter that only requires matching of thevoltage ratings. It is an object of this invention to provide such aconstant voltage dimmable LED driver.

SUMMARY OF THE INVENTION

In one aspect, the current invention provides a circuit and a method foradapting an AC input to drive a load, which include a measuring circuitfor obtaining an averaged measure of the voltage at the AC input and apower converting module for converting the AC input into a continuoustrain of pulses for driving the load, wherein the continuous train ofpulses having a duty cycle controlled by the averaged measure of thevoltage at the AC input obtained by the measuring circuit. The load canbe one or more LEDs. The AC input can be obtained from an AC powersource optionally through a dimmer.

The averaged measure of the voltage at the AC input can be approximatelyproportional to a simple average of the absolute value of the voltage atthe AC input.

The power converting module can include an AC-DC converter and a switchfor turning on and off a current path between the AC-DC converter andthe load with a duty cycle controlled by the averaged measure of thevoltage at the AC input obtained by the measuring circuit.

The switch for turning on and off the current path between the AC-DCconverter and the load can be controlled by a pulse width modulator thatoscillates at a substantially fixed frequency above human's flickeringperception threshold and has a pulse width controlled by the averagedmeasure of the voltage at the AC input obtained by the measuringcircuit.

The pulse width modulator can include a first voltage comparator orop-amp for controlling charging and discharging of a capacitor tocreate, at the capacitor, a continuously running voltage waveform thatramps up and down corresponding to charging and discharging of thecapacitor and a second voltage comparator or op-amp for comparing thevoltage at the capacitor with a voltage representing the averagedmeasure obtained by the measuring circuit, wherein the output of thesecond voltage comparator or op-amp is used to control the switch forturning on and off the current path between the AC-DC converter and theload.

The AC-DC converter can operate stably over a wide range of the AC inputto produce a substantially constant voltage output and, as the voltageat the AC input is gradually reduced, the duty cycle gradually reducesand can reach 0% slightly before the voltage at the AC input reaches thelow end of the stable operating range of the AC-DC converter.

The AC-DC converter can include a rectifier, an isolation diode, and aDC-DC converter, wherein the input of the rectifier is connected to theAC input, the output of the rectifier is connected to the anode of theisolation diode, the cathode of the isolation diode is connected to theinput of the DC-DC converter, and the measuring circuit obtains itsinput from the output of the rectifier before the isolation diode.

The circuit and the method for adapting an AC input to drive a load canfurther include a bleeder for providing minimum holding current for thedimmer. The bleeder does not have to be perfect and can have a smallamount of premature termination of the holding current and pulse topulse variations.

According to the current invention, a constant voltage type of dimmableLED driver can be created by adding a few components to the design of aconventional AC-DC converter without or with very little modifications.Therefore, the well developed and perfected art of making conventionalAC-DC converters can be leveraged to quickly create such a dimmable LEDdriver.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of the current invention, which can be madeby adding a few components to a conventional AC-DC converter modulewithout modification.

FIG. 2 shows one example implementation details of the embodiment shownin FIG. 1.

FIG. 3 shows another embodiment of the current invention, which can bemade by slightly modifying the design of a conventional AC-DC converter.

FIG. 4 shows one example implementation of the embodiment shown in FIG.3.

FIG. 5 shows another example implementation of the embodiment shown inFIG. 3.

FIG. 6 shows an example design of a DC-DC converter that can be used inthe circuits of FIG. 4 and FIG. 5.

FIG. 7 shows some waveforms to demonstrate how the Pulse Width Modulator(106) and (307) can be implemented using two voltage comparators.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows how a dimmable LED driver can be made from a conventionalAC-DC converter module by adding a few components. As shown in FIG. 1,the power from the AC power source (101) passes through Dimmer (102) andis fed into the input of the dimmable LED driver (100) made according tothe current invention. The output of the dimmable LED driver (100) isconnected to LED (108). Dimmer (102) can be any type of dimmer that canbe used for dimming an incandescent lamp, including leading edge phasedimmers, trailing edge phase dimmers, or VARICs (variable output voltagetransformers), as long as the average voltage fed into the dimmable LEDdrive (100) can be gradually reduced under user control. LED (108) canbe one LED or any type of arrangement of LED arrays, including flexibleLED strips or other type of LED lighting fixture that can be driven by aspecific DC voltage (for example, 12V).

The dimmable LED driver (100) can be made from an AC-DC converter module(104) by adding additional components including Bleeder (103), AverageVoltage Sensor (105), Pulse Width Modulator (106), and PWM Switch (107).The AC-DC converter module (104) needs to be able to accept wide rangeof input voltages and output a constant or nearly constant voltage thatmatches the specified voltage rating of the LED (108). For example, a socalled “universal input” power supply that is specified for 85V-264V ACinput range can be used as the wide input range AC-DC converter (104).Dimmable LED driver (100) can be made by using an off-the-shelf AC-DCconverter module (104) separately manufactured (or purchased from avendor) and adding the rest of the components (103), (105), (106), (107)fabricated on a separate board. It can also be fabricated as one boardthat includes the design of an AC-DC converter module (104) (which canbe directly copied from a well developed and perfected conventionalAC-DC converter without modification) and the rest of the components(103), (105), (106), (107) designed according to the current invention.Bleeder (103) is used for holding a minimum current for certain type ofdimmer that require a minimum holding current. The Bleeder (103) can beany type of passive bleeder, such as a simple resistor, or any type ofactive bleeder, such as one that is turned on only when the AC voltageis below certain level or when the AC-DC converter (104) does not drawenough current for holding a TRIAC dimmer conductive. If the dimmer(102) does not require a minimum holding current, Bleeder (103) can beignored. Average Voltage Sensor (105) is used to measure the averagevoltage at the output of the dimmer (102). The average is taken overmany cycles of the AC power frequency, typically 50-60 Hz. Forconvenience of discussions, the term “average voltage” is used hereinand throughout this specification to mean taking average of the absolutevalue of the voltage without regarding the positive/negative signs(otherwise, the average of a symmetrical AC signal is always 0). Alltypes of dimmers output an average voltage corresponding to the dimminglevel, and therefore the average voltage measured by the Average VoltageSensor (105) represents the dimming level. The average voltage measuredby the Average Voltage Sensor (105) is used to control the Pulse WidthModulator (106). The Pulse Width Modulator (106) generates a continuoustrain of pulses whose duty cycle is controlled by the average voltagemeasured by the Average Voltage Sensor (105). The Pulse Width Modulator(106) should be running at a frequency above human's flicking perceptionthreshold (for example 2 kHz) independent of the frequency of the ACpower source (101). The output of the Pulse Width Modulator (106) isused to control the PWM Switch (107) to turn on and off the connectionbetween the AC-DC converter (104) and the LED (108). In this way, as auser adjusts the dimmer (102) to dim the light, the average voltage atthe dimmer output drops reducing the fraction of time (i.e. the dutycycle) the power supplied to the LED (108) is turned on. Because the LED(108) is turned on and off at a frequency higher than the threshold ofhuman perception. it will appear that the LED (108) is simply dimmedwithout any flickering.

Care should be taken in designing the Pulse Width Modulator (106) toensure that the duty cycle can reach 0% (constantly off) before theaverage AC voltage drops to a level that the AC-DC converter (104) doesnot work anymore. For example, if the AC-DC converter (104) stopsworking when the average AC voltage drops below 20V, then the PulseWidth Modulator (106) can be designed to reach 0% duty cycle (constantlyoff) when the average AC voltage drops to a level slightly higher than20V. It is also reasonable to leave some margin on the high voltage sideto ensure that the duty cycle can reach 100% (constantly on) slightlybefore the dimmer output reaches 100% of the input, because there arealways some variations in the AC voltage of the power lines and somedimmers never reach 100% of the input. An example design to be used withnorth American's 120V/60 Hz power utility is that the duty cycle of thePulse Width Modulator (106) will change from 100% to 0% when the averageAC voltage at the output of the dimmer (102) changes from 96V to 24V.(Please note, for a sine wave: Average=(2√/2/π) RMS. Therefore 96Vaverage voltage corresponds to 106.6V RSM, leaving about 11% margin onthe high side.)

Applicant has observed that although a universal input AC-DC converteris typically specified to work for an input range of 85V to 264V, it canstill work at much lower input voltage levels when used as described inthe current invention. The reason is because the 85V limit is specifiedas the lower limit that the AC-DC converter can still provide the fullpower. When used in the current invention, however, if the input voltageis very low, the duty cycle that drives the LED (108) is also very low,and therefore the AC-DC converter only needs to provide very littlepower. Another reason is that a typical AC-DC converter has a bridgerectifier in the front, followed by a low pass filter with a largecapacitor before feeding the rectified signal to a DC-DC converter. Therectifier and the large capacitor following it effectively forms a peakholder when the load current is very low. Therefore, even if the averagevoltage is very low, the voltage fed into the DC-DC converter iseffectively the peak voltage which can be significantly higher than theaverage voltage, especially when the signal is dimmed by a phase cutdimmer. For these reasons, the lower limit of the average input voltagecan usually be extended to a much lower level than the specified inputvoltage range. Applicant has observed that a typical universal inputAC-DC converter specified to work over an input voltage range of 85V to264V still works for an average input voltage as low as 20V when used inthe current invention. If a conventional universal input AC-DC converterdoes not work at very low voltage level when used in the currentinvention, it is usually easy to slightly modify the design of theconverter to make it work. One skilled in the art will be able to do theappropriate modification.

FIG. 2 shows the details of an example implementation of the embodimentshown in FIG. 1. While the example contains many specific details, thesespecific details should not be construed as limiting the scope of theinvention. One of ordinary skills in the art will recognize that theinvention can be implemented in variety of ways different from theexample shown herein.

In this example, the Bleeder (103) is simply a resistor R1. One ofordinary skills in the art will recognize that the Bleeder (103) can beimplemented in variety of ways as long as a minimum holding current ismaintained when the AC-DC converter (104) does not draw enough current.The Average Voltage Sensor (105) directly measures the average ACvoltage at the output of dimmer (102). The AC voltage at the output ofthe dimmer (102), through the resistor R2, causes an AC current at theinput side of the optoisolator VO1 and produces a proportional DCcurrent on the output (phototransistor) side. Note that an ACoptoisolator such as FOD814 made by Fairchild Semiconductor allows ACinput and therefore allows the average voltage to be directly measuredfrom the AC source without the need of a rectifier. The current on theoutput side of VO1 is proportional to the voltage at the output of thedimmer and fluctuates with it. However, because there is a largecapacitor C1 in parallel with resistor R3 which will be charged when theoutput current of VO1 is higher than average and discharged when it islower than average, the current that flows though resistor R3 isessentially the average output current of VO1. Therefore, the voltageacross R3 is essentially proportional to the average AC voltage at theoutput of the dimmer (102), but will be limited by the VCC (12V). Thevoltage across resistor R3, which is the output of the Average VoltageSensor (105), can be approximately calculated as (rR3/R2)<V_(ac)> whenthe result is less than VCC, but will be limited to VCC if the result isgreater than VCC (the saturation voltage of the phototransistor in VO1,typically 0.2-0.5V, is ignored), where <V_(ac)> is the average ACvoltage at the output of the dimmer (102) and r is the current transferratio of the optoisolator VO1. In the example implementation of FIG. 2,VO1 has a current transfer ratio of 55%. Therefore, the voltage acrossR3 is calculated to be about 10% of the average AC voltage at the outputof the dimmer (102). When the average AC voltage at the output of thedimmer (102) changes from 96V to 24V, the output of the Average VoltageSensor (105) changes from 9.6V to 2.4V. Because the current transferratio of optoisolators has large variations, R3 may need to be adjustedto match the optoisolator VO1. In large volume productions, it may benecessary to pre-sort optoisolators into different bins of roughly thesame current transfer ratio to be used in the same batch of production.In this way, R3 can be fixed for the batch. This problem can be avoidedif optoisolator is not used in the Average Voltage Sensor, as shown inthe example of FIG. 5. which will be discussed later.

As shown in FIG. 2, the Pulse Width Modulator (106) can be made usingtwo voltage comparators. (Op-amps can also be used instead ofcomparators, although op-amps are optimized for linear operations whilelinearity is not necessary here.) One comparator U1-B is used to form anoscillator. It works as the following: When the output of U1-B is high,the voltage at the “+” input of comparator U1-B will be pulled up byresistors R7 and R5 against R4 to about 80% of the VCC while capacitorC2 will be charged through resistor R6. As the capacitor C2 is chargedto a voltage exceeding the voltage at the “+” input of U1-B (80% ofVCC). the output of U1-B will reverse and become low. When the output ofU1-B is low, the “+” input of U1-B will be pulled down by resistor R7and R4 against R5 to about 20% of VCC and capacitor C2 will bedischarged through resistor R6. When capacitor C2 is discharged to avoltage below the “+” input of U1-B (20% of VCC), the output of U1-Bwill reverse and become high again, and the cycle continues. In thisway, the voltage across capacitor C2 will oscillate between 20% of VCCand 80% of VCC creating a waveform shown in the upper part of FIG. 7.Such a waveform is fed into the “−” input of comparator U1-A while the“+” input of U1-A is connected to the output of the Average VoltageSensor (105). The output voltage of the Average Voltage Sensor (105) isrepresented by the dotted horizontal line in the upper part of FIG. 7.When the “+” input of U1-A is higher than the “−” input, the output ofU1-A will be high, and vice versa, creating a waveform shown in thelower part of FIG. 7, in which the pulse width is equal to the time thevoltage across capacitor C2 is below the output of the Average VoltageSensor (105). In this way, as the output of the Average Voltage Sensor(105) changes from 20% of VCC (about 2.4V) to 80% of VCC (about 9.6V),the duty cycle of the output of the Pulse Width Modulator (106) changesfrom 0% to 100%. The output of the Pulse Width Modulator (106) is usedto drive the PWM Switch (107), which can be an N-Channel MOSFET Q1, toturn on and off the current path between the AC-DC converter module(104) and the LED (108). Q1 should be chosen to support large currentand have very low resistance when turned on. For example, FDP65N06 madeby Fairchild Semiconductor has an on resistance of less than 0.016Ω, andtherefore at 5 A current, the voltage drop across Q1 is less than 0.08Vand the power dissipation is less than 0.4W. No heat sink is needed. TheAC-DC converter module (104) used in this example is an off-the-shelf 65W 12V power supply VOF-65-12 made by V-Infinity. The 12V output of thepower supply is also used to power the VCC of the Pulse Width Modulator(106) and part of the Average Voltage Sensor (105).

FIG. 3 shows another embodiment of the current invention, which can bemade from a conventional AC-DC converter with minor modifications. Twodetailed implementation examples for the embodiment of FIG. 3 are shownin FIG. 4 and FIG. 5. As shown in FIG. 3, a typical conventional wideinput range AC-DC converter already includes Rectifier (303) and WideInput Range DC-DC Converter (305) directly connected. The modificationincludes cutting the original connection and inserting the IsolationDiode (304) between the Rectifier (303) and the Wide Input Range DC-DCConverter (305), and adding some components including Bleeder (302),Average Voltage Sensor (306), Pulse Width Modulator (307), and PWMSwitch (308). The modification can be made by literally cutting thetraces on an already manufactured AC-DC converter and adding theadditional components (302), (304), (306), (307), and (308) on aseparate board, but more preferably, the design can be modified with theadditional components fabricated on the same board. Bleeder (302) isconnected to the output of the Rectifier (303) to sink rectified DCcurrent. An AC bleeder, similar to Bleeder (103) shown in FIG. 2 canalso be used to sink AC current instead of, or in addition to, Bleeder(302), and the design still works. In the example implementations shownin FIGS. 4 and 5, Bleeder (302) is a passive bleeder formed by theresistors R1, R2 and capacitor C1.

Applicant observed that the passive bleeder formed by R1, R2 andcapacitor C1 of FIG. 4 and FIG. 5 does not absolutely prevent prematureshut-off of a TRIAC dimmer caused by lack of minimum holding current.When the AC voltage is high or moderate, C1 will be charged through R2,and the current charging capacitor C1, along with the current throughR1, will provide enough holding current. However, when the AC voltagebecomes very low. C1 will be discharged through resistors R2 and R1 inseries. This discharging current can raise the voltage across R1 and itis possible that the voltage across R1 becomes higher than the ACvoltage to make the rectifier reverse biased. When this happens, thecurrent will be 0 and the TRIAC dimmer will be shut off prematurely. Theeffect is a small portion of the trailing edge of the AC waveform willbe cut off. Applicant also observed that, at some dimming levels, such apremature cut-off can vary from cycle to cycle. For many other types ofdimmable LED drivers, this would cause serious flicking problems.However, when used in the current invention, no serious harm is causedby such a premature shut-off. First, because a conventional AC-DCconverter only has current flow through the rectifier when the ACvoltage is near its peak anyway, cutting off a small tail of the ACwaveform has no effect on the amount of power the converter is getting.Additionally, because the voltage used to control the Pulse WidthModulator (307) is averaged over many cycles of the AC frequency, thevariations from cycle to cycle are smoothed out producing virtuallyconstant voltage at the output of Average Voltage Sensor (306). The onlyeffect left is that, because an additional trailing edge is cut, thedimming level is slightly shifted (for example, when the dimmer isadjusted to a nominal 50% dimming level, it will actually be 40%). Thiswill not cause any serious problem. If enough margin on the top of thedimming range is designed as describe above, the dimmer can stillcontrol the duty cycle of the Pulse Width Modulator (307) to smoothlychange from 0% to 100%. This example shows that a weaker and lessperfect bleeder that may not work in some other types of dimmable LEDdrivers can be used in the current invention without causing seriousproblems. Using a weaker and less perfect bleeder has the advantage ofreduced power consumption and simplicity.

Continuing the descriptions of FIG. 3, Isolation Diode (304) can simplybe a diode, as shown in the implementation examples of FIG. 4 and FIG.5. The reason for inserting an isolation diode is because a typicalAC-DC converter usually has a large capacitor in front of the DC-DCconverter following the rectifier (see C4 in FIG. 4 and FIG. 5 and C1 inFIG. 6). If the output of the rectifier is directly connected to thiscapacitor, they will form a peak holder, and the voltage at the outputof the rectifier will be held near the peak of the rectified AC voltage,preventing the Average Voltage Sensor (306) from correctly measuring theaverage voltage. Inserting Isolation Diode (304) will isolate Rectifier(303) from this large capacitor and allow the output of the rectifier torise and fall along with the AC voltage so that the Average VoltageSensor (306) can get accurate measurement. In addition, the isolationdiode and the large capacitor in front of the DC-DC converter still forma peak holder allowing the working range of the converter to be extendedto very low average voltage levels, as discussed previously. The AverageVoltage Sensor (306), the Pulse Width Modulator (307), and the PWMSwitch (308) are essentially the same as the components (105), (106),and (107) in FIG. 1, with some minor design differences as shown in FIG.4 and FIG. 5.

In the example implementation shown in FIG. 4, the Average VoltageSensor (306) is basically the same as the Average Voltage Sensor (105)of FIG. 2, except that the rectified signal, instead of the AC signal issensed. Therefore, a DC optoisolator such as FOD817 is used for VO1,instead of an AC optoisolator such as FOD814. The Pulse Width Modulator(307) and PWM Switch (308) shown in FIG. 4 are identical to thecorresponding components (106) and (107) in FIG. 2.

In the implementation example shown in FIG. 5, however, the isolationboundary is not inside the Average Voltage Sensor (306) but is at theoutput of the PWM Switch (308). In addition, an auxiliary power supplyprovided by the DC-DC converter (305), which can be different from theconverter output, is used to power the Pulse Width Modulator (307). Inthis example, the output of the Average Voltage Sensor (306) can becalculated as

$\frac{R\; 3}{\left( {{R\; 3} + {R\; 4}} \right)}$<V_(rect)>, where <V_(rect)> is the average voltage at the output of theRectifier (303) which is roughly the same as the average AC voltage atthe output of the dimmer (102). Because optoisolator is not used here,R3 and R4 can be fixed and does not have to be matched with the currenttransfer ratio of an optoisolator, and therefore, the problem discussedearlier with respect to FIG. 2, which also exists in the implementationof FIG. 4, can be avoided. Using the above formula and R3=20 kΩ, R4=130kΩ, when the average voltage is changed by the dimmer (102) from 96V to24V, the output voltage of the Average Voltage Sensor (306) will changefrom 12.8V to 3.2V, which corresponds to 80% and 20% of the auxiliary16V power supply respectively. The Pulse Width Modulator (307) is thesame as the Pulse Width Modulator (106) of FIG. 2, except it is poweredby 16V instead of 12V. The voltage across capacitor C2 will be the samewaveform shown in the upper part of FIG. 7, except that it oscillatesbetween 20% and 80% of 16V (between 3.2V and 12.8V). Therefore, as thedimmer (102) changes the average voltage from 96V to 24V, the duty cycleof the output of the Pulse Width Modulator (307) will change from 100%to 0%. The output of the Pulse Width Modulator (307) is used to drivethe PWM Switch (308) through the optoisolator VO1. Here, the variationof the current transfer ratio of the optoisolator is not an issue.

FIG. 6 shows the DC-DC converter part of a typical wide input rangeAC-DC, converter that can be used as component (305) in both theimplementations of FIG. 4 and FIG. 5. It also shows how the auxiliarypower supply for the Pulse Width Modulator (307) is obtained. A typicalAC-DC converter already has an auxiliary power supply derived from anauxiliary winding of the flyback transformer to power the controller IC(such as U1 shown in FIG. 6). This auxiliary power can be used forproviding the auxiliary power for the Pulse Width Modulator (307).However, one needs to be careful, because the controller IC usuallyrequires a small startup current obtained directly from the rectified ACsource through a resistor such as R4 shown in FIG. 6. If the auxiliarypower for the Pulse Width Modulator (307) is obtained directly from theVCC of the controller IC U1, the additional current drawn by the PulseWidth Modulator (307) may prevent the resistor R4 from pulling up theVCC of U1 to a high enough voltage to get started. Therefore, theauxiliary power should be obtained from the auxiliary windings andrectified with an additional diode D5, as shown in FIG. 6. Of course, ifthe current provided by R4 is enough to supply both the Pulse WidthModulator (307) and start the controller IC U1, then the auxiliary powerfor the Pulse Width Modulator (307) can be obtained directly from theVCC of U1. It is also possible to modify R4 to a smaller value toprovide enough current to supply both the Pulse Width Modulator (307)and start the controller IC U1.

The above examples show how a constant voltage dimmable LED driver canbe created by adding a few components to the design of a conventionalAC-DC converter without or with very little modifications. Themodifications and the additional components incur very little additionalcost, require very little board space, and do not change the basicelectrical and thermal characteristics of the original design of theAC-DC converter. All the over current, over voltage, and overtemperature protection mechanisms built in the original AC-DC converterstay intact. Therefore, the well developed and perfected art of theconventional AC-DC converter can be leveraged to quickly create aconstant voltage dimmable LED driver according to the current invention.

While this invention has been described in terms of severalimplementations, it is contemplated that alterations, modifications andpermutations thereof will become apparent to those skilled in the artupon a reading of the specification and study of the drawings. Forexample, while the bleeders demonstrated in the example implementationsare passive bleeders, those skilled in the art will recognize thatvarious active bleeders can be used, including bleeders that are turnedon only when the AC voltage falls below a certain level or when theAC-DC converter does not draw enough current to hold a TRIAC dimmerconductive. Such bleeders may waste less power than passive bleeders. Inaddition, a power factor corrected (PFC) AC-DC converter can be used inplace of the conventional AC-DC converter demonstrated in the exampleimplementations. A power factor corrected AC-DC converter will behavelike a resistor and will draw current even when the AC voltage is verylow, unlike a conventional AC-DC converter that only draws current whenthe AC voltage is near its peak. A PFC converter may draw enough currentat low AC voltages to hold a TRIAC dimmer conductive and therefore thebleeder can be much weaker or even eliminated. Additionally, averagevoltage described above is only one measure of the AC voltage. Othertype of measures can be used. For example, the Average Voltage Sensorcan be replaced by a circuit that measures RMS voltage instead of theaverage voltage. In fact, any type of averaged measure of the voltage atthe output of the dimmer can be used and the invention still works.Although the Pulse Width Modulator described above has a fixed frequencywhile the duty cycle is changed by changing the pulse width, a differentdesign can be used in which both the frequency and the pulse width canchange, as long as the frequency stays above the threshold of human'sflickering perception. Non-linearity can be built into either theAverage Voltage Sensor or the Pulse Width Modulator or both to make thedimming curve to appear more smooth to human perception (humanperception for brightness is nonlinear). While in the exampleimplementations described above, the duty cycle of the Pulse WidthModulator can be changed from 0% to 100%, a different design can be madeto limit the maximum duty cycle to be less than 100%, for example 50%.In this case, the output voltage of the AC-DC converter can be designedto be higher than the nominal voltage rating of the LED so that theaverage current through the LED can be the same as if the LED is drivenwith the rated voltage with 100% duty cycle. For example, a typical LEDarray rated 12V can be driven by a pulse train of 13V and 50% duty cycleto have roughly the same average current as if it is driven by 12Vconstantly on. In this way, the dimming level can still be changed from0% to 100% but the duty cycle can be limited to be less than 100%. Whilethe above descriptions primarily use LED lighting as an example, theinvention can be used for driving other types of load, for example ahalogen light. All these variations and many other possible variationsnot listed here will become apparent to those skilled in the art upon areading of the specification and study of the drawings.

Furthermore, certain terminology has been used for the purposes ofdescriptive clarity, and should not be construed to limit the invention.Some numerical values of various parameters described above are onlyexamples and should not be construed to limit the scope of theinvention. One skilled in the art will recognize that different valuesmay be used and the invention will still work. It is therefore intendedthat the following appended claims include all such alterations,modifications and permutations as fall within the true spirit and scopeof the present invention.

I claim:
 1. A circuit for adapting an AC input to drive a loadcomprising: a) a measuring circuit for obtaining an averaged measure ofthe voltage at the AC input; b) a power converting module, comprising anAC-DC converter and a switch for turning on and off a current pathbetween the AC-DC converter and the load, for converting the AC inputinto a continuous train of pulses for driving the load, with a dutycycle controlled by the averaged measure of the voltage at the AC inputobtained by the measuring circuit, wherein the switch for turning on andoff the current path between the AC-DC converter and the load iscontrolled by a pulse width modulator oscillating at a substantiallyfixed frequency above human's flickering perception threshold and havinga pulse width controlled by the averaged measure of the voltage at theAC input obtained by the measuring circuit.
 2. The circuit of claim 1wherein the load is an array of at least one LED.
 3. The circuit ofclaim 1 wherein the AC input is obtained from an AC power source througha dimmer.
 4. The circuit of claim 3 further including a bleeder forproviding minimum holding current for the dimmer, wherein the bleedercan be an imperfect bleeder that can have a small amount of prematuretermination of the holding current and pulse to pulse variations.
 5. Thecircuit of claim 1 wherein the pulse width modulator comprising a firstvoltage comparator or op-amp for controlling charging and discharging ofa capacitor to create, at the capacitor, a continuously running voltagewaveform that ramps up and down corresponding to charging anddischarging of the capacitor and a second voltage comparator or op-ampfor comparing the voltage at the capacitor with a voltage representingthe averaged measure obtained by the measuring circuit, wherein theoutput of the second voltage comparator or op-amp is used to control theswitch for turning on and off the current path between the AC-DCconverter and the load.
 6. The circuit of claim 1 wherein the AC-DCconverter can operate stably over a wide range of the AC input toproduce a substantially constant voltage output and, as the voltage atthe AC input is gradually reduced, the duty cycle gradually reduces andcan reach 0% before the voltage at the AC input reaches the low end ofthe stable operating range of the AC-DC converter.
 7. The circuit ofclaim 1 wherein the AC-DC converter comprising a rectifier, an isolationdiode, and a DC-DC converter, wherein the input of the rectifier isconnected to the AC input, the output of the rectifier is connected tothe anode of the isolation diode, the cathode of the isolation diode isconnected to the input of the DC-DC converter, and the measuring circuitobtains its input from the output of the rectifier before the isolationdiode.
 8. The circuit of claim 1 wherein the averaged measure of thevoltage at the AC input is approximately proportional to a simpleaverage of the absolute value of the voltage at the AC input.
 9. Amethod for adapting an AC input to drive a load comprising: a) obtainingan averaged measure of the voltage at the AC input; b) using a powerconverting module comprising an AC-DC converter and a switch for turningon and off a current path between the AC-DC converter and the load toconvert the AC input into a continuous train of pulses for driving theload, with a duty cycle controlled by the averaged measure of thevoltage at the AC input, wherein the switch for turning on and off thecurrent path between the AC-DC converter and the load is controlled by apulse width modulator oscillating at a substantially fixed frequencyabove human's flickering perception threshold and having a pulse widthcontrolled by the averaged measure of the voltage at the AC inputobtained by the measuring circuit.
 10. The method of claim 9 wherein theload is an array of at least one LED.
 11. The method of claim 9 whereinthe AC input is obtained from an AC power source through a dimmer. 12.The method of claim 11 further including using a bleeder for providingminimum holding current for the dimmer, wherein the bleeder can be animperfect bleeder that can have a small amount of premature terminationof the holding current and pulse to pulse variations.
 13. The method ofclaim 9 wherein the pulse width modulator comprising a first voltagecomparator or op-amp for controlling charging and discharging of acapacitor to create, at the capacitor, a continuously running voltagewaveform that ramps up and down corresponding to charging anddischarging of the capacitor and a second voltage comparator or op-ampfor comparing the voltage at the capacitor with a voltage representingthe averaged measure obtained by the measuring circuit, wherein theoutput of the second voltage comparator or op-amp is used to control theswitch for turning on and off the current path between the AC-DCconverter and the load.
 14. The method of claim 9 wherein the AC-DCconverter can operate stably over a wide range of the AC input toproduce a substantially constant voltage output and, as the voltage atthe AC input is gradually reduced, the duty cycle gradually reduces andcan reach 0% before the voltage at the AC input reaches the low end ofthe stable operating range of the AC-DC converter.
 15. The method ofclaim 9 wherein the AC-DC converter comprising a rectifier, an isolationdiode, and a DC-DC converter, wherein the input of the rectifier isconnected to the AC input, the output of the rectifier is connected tothe anode of the isolation diode, the cathode of the isolation diode isconnected to the input of the DC-DC converter, and the measuring circuitobtains its input from the output of the rectifier before the isolationdiode.
 16. The method of claim 9 wherein the averaged measure of thevoltage at the AC input is approximately proportional to a simpleaverage of the absolute value of the voltage at the AC input.